I've separated my front bumper and now put it in the same collection but both objects are giving different shading between them
Collections aren’t for managing materials.
Go to the Materials tab of the Properties editor instead:
Then, check that the same shader is set for both the hood and the other thing (select each in turn and observe what is displayed in their material slots).
For more information, please see the Materials information in the Blender Manual, particularly Material Assignment.